The invention relates to a photomultiplier tube and particularly to a photomultiplier tube having at least one Nichrome dynode for reducing and stabilizing the anode sensitivity and gain of the tube.
In the manufacture of photomultiplier tubes it is desirable to have a photocathode which is highly sensitive to visible and near infrared light in order to obtain a high signal to noise ratio. It is also desirable to simultaneously maximize the sensitivity of both the photocathode and a plurality of dynodes to achieve high anode current gain. Method of sensitizing electron emissive surfaces of, for example, base layers of antimony on nickel substrates are well known in the art of electron discharge devices. Photoemissive materials and techniques relating thereto are, for example, described in "Photoemissive Materials" by A. H. Sommer, John Wiley and Sons, Inc., New York, 1968 and is herein incorporated by reference.
A method for simultaneously sensitizing the antimony layers of a photocathode and a plurality of dynodes having a supporting nickel substrate is described in U.S. Pat. No. 4,002,735 issued to McDonie et al. on Jan. 11, 1977, entitled, "Method of Sensitizing Electron Emissive Surfaces of Antimony Base Layers with Alkali-Metal Vapors", incorporated by reference herein.
A structure which provides a "buffer" layer between the nickel electrode substrate and the antimony base layer is described in U.S. Pat. No. 4,039,887 issued to McDonie on Aug. 2, 1977, entitled "Electron Emitter Including Porous Antimony", and incorporated by reference herein. The disclosed structure prevents alloying between the nickel substrate and the antimony base layer and provides an electrode which, when sensitized with alkali metal vapor, has the desired sensitivity.
An alternative method for preventing alloying between the nickel substrate of an electron emissive electrode and a base layer of antimony by forming an aluminum oxide film between the substrate and the base layer is disclosed in U.S. Pat. No. 4,160,185 issued to Tomasetti et al. on July 3, 1979, entitled, "Red Sensitive Photocathode Having an Aluminum Oxide Barrier Layer", and incorporated by reference herein.
The above-mentioned patents to McDonie et al., McDonie, and Tomasetti et al. have yielded photomultiplier tubes having both high photocathode sensitivity and high anode sensitivity. It has been noted however, that in some of the photomultiplier tubes, incorporating the above-described improvements, excessive anode sensitivity, i.e., anode sensitivity exceeding the permissible maximum manufacturing limit, and gain instability has been encountered. In photomultiplier tubes such as the RCA 4840 which has a multialkali photocathode comprising sodium, potassium, cesium, and antimony, and a plurality of alkali antimonide dynodes, the problem of excessive anode sensitivity and unstable gain has been pronounced. The alkali antimonide dynodes in this tube structure have effective secondary emission gains ranging from 3.5 to 8 per electrode stage. It is believed that the gain instability is caused by a change in the chemical equilibrium of the secondary emissive surface of the electrode. Such a change can be caused by either the addition or depletion of alkali materials from the photoemissive surfaces due, for example, to electron bombardment of the photoemissive surfaces, or the migration of alkali-vapors within the tube.
Since the above-mentioned tubes can only be economically produced by a batch sensitizing process requiring evaporation of excess quantities of alkali metals followed by baking the tubes to remove excess alkali materials in order to achieve the desired sensitivity, any solution to the gain problem cannot alter the existing sensitizing process.